Despite having the same genetic makeup, identical twins have their own distinctive personalities. Just how their individuality emerges has remained a bit of a mystery. But now, researchers have found that life experiences affect brain development — and this may help us understand how personalities form.

Who we are behaviorally and physically depends on a number of factors. Genetics play a huge role, but so does the environment, which can influence the way our genes are expressed. For example, identical twins may have different heights or weights because of dietary differences, serious illnesses or even differing placental connections while still in the womb.

"In twin studies it had been clear that even though the twins are identical (monozygotic), there are still some differences between them that emerges over time," says Gerd Kempermann, a behavioral geneticist at the Dresden University of Technology and the German Center for Neurodegenerative Disease in Germany. "Identical twins are often amazingly similar, but mothers and close relatives can still tell them apart easily."

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Importantly, identical twins raised in the same household — the same "outer" environment — still develop personality differences over time. Behavioral geneticists have long pegged these differences to influences by the "non-shared environment," though there's no real consensus on exactly what the non-shared environment consists of, Kempermann told io9. In twin studies, he says, these non-shared environmental influences essentially boil down to the individual experiences siblings have, and their own personal interactions with their environment.

Kemperman and his colleagues studied genetically identical mice, and found that their experiences influenced the growth of new neurons in the hippocampus — a part of the brain associated with learning and memory. They believe these neurological changes promote individual differences in behavior and personality.

Naturally, the development of personality differences should be reflected in the brain. In the hippocampus, new neurons are constantly being generated in a process called neurogenesis. Knowing this, Kempermann and his colleagues wondered: How do life experiences drive individualization in the brain? Specifically, if genetically identical mice lived in the same environment, how much individuality would they each develop, and would their different "personalities" be reflected in their hippocampal neurogenesis?

To find out, Kempermann and his colleagues housed 40 genetically identical inbred mice in a complex arena. The cage had several levels, each of which was filled toys, tubes and other objects to enrich the environment. The team fitted each adult mouse with a radio-frequency identification (RFID) transponder and scattered 20 radio antennas throughout the big arena.

Schematic for the mice's multi-tiered home. Courtesy of Science/AAAS.

This setup allowed the researchers to track how much of the environment each rodent covered and quantify their exploratory behavior over the 3-month-long experiment. The researchers also injected the mice with a compound that marks cells undergoing division, allowing them to track the proliferation of new hippocampal neurons.

The team found that the mice, though genetically identical, showed highly individualized explorative behaviors. They lived in the exact same arena, but they reacted to this environment differently, Kempermann says, adding that the behavioral differences only grew over time. So some mice became explorers, who roamed the environment more as the months passed, while other mice preferred to really stick to areas they knew.

And these behavioral differences showed up in the mice's hippocampal neurogenesis — mice that explored their environment more fully grew more new neurons than their less adventurous siblings. Moreover, the mice in this large arena showed more neurogenesis on average than control mice housed in smaller, less stimulating cages.

So to recap: The mice's experiences, or interactions with their environment, affected their long-term behavioral patterns and the growth of new neurons, promoting the development of distinct personalities despite their genetically identical makeup.

And given that people also undergo hippocampal neurogenesis, Kempermann thinks that the team has pinpointed the neurological foundation for human individuality. "It's a comforting thought that it's not only our genes and it's not only the outer environment, but also our experiences that count towards our individuality," he says.

Kempermann also points out that scientists now have a new model to study how different experiences help shape different personalities. "It opens a new type of approach to an old problem," he says. For example, the team focused only on the mice's interactions with their physical environment, but what role do social interactions play in hippocampal neurogenesis and behavioral developments?

For now, the researchers are interested in getting a clearer picture of their findings. One thing they'd like to figure out is how the mice's behaviors start developing. In their study, they saw that mice that were initially adventurous explored the environment more over time, until that exploratory trait became stable — but what made those mice willing to examine their surroundings in the first place?

"Then, of course, we're interested in the mechanisms," Kempermann says. "How does the activity of exploration actually change the brain? We want to address the causality of it all."